Photoacoustic detection of gases is primarily based on the analysis of the effects of absorbed electromagnetic energy on the gases. The measurement of these effects on the gases is carried out by means of acoustic detection. The electromagnetic energy involved in photoacoustic gas detection generally corresponds to optical radiation, which is known in the art to refer to electromagnetic radiation covering the wavelength range from 100 nm to 1 mm (see, e.g., standard DIN 5031).
There are gas detection applications where the size of the available gas sample to be measured is small and therefore the gas detector internal volume is limited. Moreover, there are applications where it is preferable to have a gas detector with an internal volume and a geometry which allows a gas sample to be flushed through the gas detector efficiently. In the latter case, the small internal volume should be of an elongated shape with the gas flowing from one end (inlet) towards the other end (outlet), resembling the gas flow inside a tube. The gas flow inside the detector is preferably smooth and uniform. The gas detector can, for example, be used for gas chromatography.
One design requirement for a photoacoustic detector is an acoustic resonance as a way to amplify a sound signal and therefore increase detector sensitivity. Resonant acoustic waves should be possible to produce and maintain within the detector internal volume. Typical design optimizations for improving gas detection sensitivity include larger microphones and diaphragms, dual or multiple microphones, resonant cavities with buffering volumes and larger cells and windows for maximizing radiation input.
However, these designs are not optimized for small internal volumes.
Therefore, there is a need for a photoacoustic detector design which has small internal volume and high sensitivity.